Energy storage devices, such as ultracapacitors (i.e., electrochemical capacitors, electrical double layer capacitors or supercapacitors) are increasingly important in powering a wide variety of devices such as, for example, motor vehicles, cellular telephones, computers, etc. and furthermore, may be used as a replacement for or in conjunction with conventional batteries. Ultracapacitors have a number of advantages compared to conventional batteries such as, for example, long life cycle, easy construction, short charging time, safety and high power density.
Conventional ultracapacitors include metal substrates (e.g., aluminum) on which are deposited active materials which have high surface area as the electrodes. Activated carbon is the most commonly used active material, which is typically deposited on metal substrates as a paste and forms a thin film on the surface of the substrate.
Recently, carbon nanotubes have been used as active materials in electrodes to form ultracapacitors. Similarly to activated carbon, carbon nanotubes can be deposited as a paste, which includes a binder, on metal substrates. However, deposition of carbon nanotubes as a paste leads to increased high interface resistance because of the continuing presence of the binder and poor mechanical/electrical contact between carbon nanotubes and metal collectors, which leads to poor power performance of the capacitor. Alternatively, carbon nanotubes may be grown on metal substrates with co-deposition of a metal catalyst. However, the continuing presence of the catalyst leads to poor power performance of the capacitor.
More recently, chemical vapor deposition has been used to directly grow continuous films of both vertically aligned or randomly dispersed carbon nanotubes on thick, highly polished metal substrates. Such carbon nanotubes are useful electrodes for constructing an ultracapacitor but are costly, difficult to package and/or mold and have a reduced performance profile because substantial resistance develops between the carbon nanotubes and the metal surface upon deposition of charge.